The present invention relates to a method and a system for improving conversion efficiency of a lean NOx catalyst in a diesel or lean burn gasoline engine, and, more particularly, by controlling conditions within the catalyst to cause the reductant to provide higher NOx conversion rates.
Internal combustion engines commonly rely on exhaust aftertreatment devices to convert regulated components: carbon monoxide, hydrocarbons, and nitrogen oxides (NOx), into carbon dioxide, water, nitrogen, and oxygen. Exhaust catalysts have been extensively developed to obtain high conversion efficiencies on stoichiometric exhaust gases. Stoichiometric conditions are achieved when the fuel and oxidizer supplied to the engine is in a proportion which, if reaction of the fuel were complete, produce carbon dioxide, water, and nitrogen. It is known to those skilled in the art, though, that higher fuel efficiency is obtained from engines operating at air-fuel ratios lean of stoichiometric, that is, with an excess of air. These lean burning engines may be diesel engines, stratified-charge gasoline engines in which the fuel and air are only partially mixed, and homogeneous-charge, lean-burn gasoline engines in which the fuel and air are mostly premixed prior to combustion. Because of the desire for high fuel efficiency, lean burning engines are in production and continue to be developed. It is known to those skilled in the art to use a NOx catalyst and continuously supply reductant to the catalyst to convert NOx while operating lean.
The problems with prior art methods are that some of the reductant supplied to the catalyst slips through the catalyst unreacted and NOx conversion is too low at temperatures below about 250xc2x0 C.
The inventors herein have recognized a method which causes the reductant that is supplied to the catalyst to be stored in such a way that the NOx conversion efficiency is improved in the lower temperature range. By obtaining higher conversion efficiency, less reductant need be supplied to the catalyst and less reductant slips through the catalyst.OLE_LINK4
Disadvantages of prior art approaches are overcome by a method for increasing the conversion efficiency of an exhaust gas component in a catalyst receiving exhaust gases from an internal combustion engine operating at a lean air-fuel ratio in which it is determined that at least a predetermined quantity of reductant is stored in the catalyst. In response, a set of operating conditions, which cause the catalyst temperature to attain a temperature above a predetermined temperature, are created. Preferably, the method further includes determining that the conversion efficiency of the exhaust gas component in the catalyst is lower than a predetermined conversion efficiency. The operating conditions leading to the catalyst temperature attaining a temperature greater than the predetermined temperature are created in response to the determination of the reductant stored in the catalyst exceeding the predetermined quantity and the determination that the conversion efficiency of the exhaust gas component in the catalyst is lower than the predetermined conversion efficiency.
A primary advantage of the present invention is that reductant supplied to the catalyst can be caused to provide a higher NOx conversion rate than known in the prior art. Specifically, the inventors of the present invention theorize that reductant is stored within the catalyst at both active and inactive sites. By providing specific conditions within the catalyst, the reductant diffuses to and is absorbed on active sites. The subsequent conversion efficiency of the lean NOx catalyst is substantially higher than heretofore possible in a lower temperature range; the higher rate is theorized to be due to reductant being stored on active sites.
Another advantage is that by practicing the present invention, the supplied reductant leads to a higher conversion efficiency of NOx which allows less reductant to be used, i.e., a more efficient use of reductant.
Another advantage of the invention herein, over prior art, is that because less reductant is supplied to the catalyst, less reductant slips through the catalyst into the tailpipe.
The above advantages and other advantages, objects, and features of the present invention will be readily apparent from the following detailed description of the preferred embodiments when taken in connection with the accompanying drawings.